Ungroup a node group while keeping parameters?
I think you can't solve this problem inside the nodeGroup itself, but you can add a value Node (for gray input factor values, RGB/vector for yellow/blue) outside of your group, plug it (from outside) to the factor input of your group and use it for tweaking and the when you ungroup, it'll keep it plugged and the value will be the same as before.

inside group


values lost when ungrouped, default values applied


values linked from outside


values saved and linked after ungrouping


As far as I know, it's not a bug, it's the normal behavior of node groups. Values are stored in the group, not in individual nodes inside the group. So when you ungroup (delete the group) all stored infos are deleted too.